Rhododendrons are at their best as garden shrubs in the Pacific Northwest, chiefly west of the Cascade range. Not only are there many of the species growing wild here, but also most of the known cultivated species do well here in a variety of locations.
Yet the genus Rhododendron is so immensely diversified that many types can be grown equally well in other parts of the country. Compared to most other garden shrubs, these beautiful plants fare better than average in the number of insect pests and other troubles afflicting them. But they are not pest-free anywhere, and every grower must know the symptoms of their common ailments and the remedies to apply.
As you go about your garden, you may stop to wonder why a certain rhododendron has not made the growth it should have. It is standing stationary, its foliage assuming a yellowish cast, perhaps remaining abnormally small or incurved as if bothered by stomach cramps. Though there is no trace of insects or other pests, the whole plant lacks normal vigor. No matter whether the season be winter, spring, or fall, these danger signals should send you at once for a shovel. Lift the entire ball of the plant out of the ground. You will find the original ball firm and intact, with very few roots beyond its confines. You will note that the earth is probably rather heavy and packed too tightly for the proper expansion of the root system. You have diagnosed the ailment; now what is the remedy?
Nine out of ten times the answer is to enlarge the hole to double the size required for the ball, digging downwards as well as sideways. Then fill this cavity with two-thirds peat moss and one-third sandy soil. Mix a good handful of cottonseed meal into this combination. Now use a small pick or pointed trowel to rip up the surface of balled roots so that a new root system can be encouraged to grow. Plant this ball into the hole you have prepared, with the surface barely a fraction of an inch below the earth level. If growing weather prevails, water the plant well. Remove as much of the heavy soil as possible, and if space remains to be filled, use peat moss or old sawdust.
Proper watering is essential in rhododendron culture. During dry weather and windy days, water the bushes once or twice a week, not just with a little sprinkling over the surface but a thorough soaking. Then wait until water is needed again. However, bear in mind that rhododendrons are benefited by spraying over the foliage. This is most beneficial if done in the evening, as that creates the right atmospheric condition for their wellbeing. Should the season be very dry, there is nothing more congenial for rhododendrons than a mulch of peat moss and lawn clippings spread over them as fertilizer. As the root system is always near the very top of the soil, do not cultivate around them as you do roses. Simply leave them alone. If weeds spring up, pull them up by hand and let them become mulch also for that produces the best natural plant food.
Here and there a rhododendron, especially one standing in a dry location, may become the prey of scale insects. If it is not treated promptly, the plant will soon look sick. Scale always appears first in spots where water seldom strikes. This fact is your cue for the first remedy to apply. Spray the plants forcefully late in the evening with a strong stream of water, trying to dislodge the scales and discourage them by wet conditions over night. Within a week or two, the plants should acquire a new look of virility, and examination should reveal the departure of the scales that were sapping the life from leaves and bark. However, if bad conditions have prevailed for a long time, it, may be well to begin your cure by spraying the bushes freely under the foliage with a solution of Vapotone, following directions on the container. This eliminates the insects very quickly and is harmless to the plants if directions are followed.
Another satisfactory method of treating scale when the insects are firmly established is to spray dormant bushes during the winter with a regular orchard lime and sulphur solution. This is a marvelous cleanup spray for all insects; their eggs, and fungus troubles as well. The drawback to using lime-sulphur is that the leaves retain a dirty appearance, discoloration from the solution, for some weeks. This discoloration eventually will disappear of its own accord.
One of the rhododendron pests most frequently met in the garden is the so-called strawberry weevil or black vine beetle, known to science as Brachyrhinus sulcatus. This beetle comes up from the ground at night and feeds on the young foliage, eating semi-circular holes on the edge of young leaves. The insect is usually about five-eighths of an inch long and one-fourth of an inch in diameter, with a body of brown or blackish color. Its presence is often overlooked for it drops to the ground in the morning, but it may be observed in the early morning on cool, cloudy days, when it delays its departure to the ground. This foliage damage is only a minor part of the injury done by this insect. In a few weeks it lays its eggs at the base of the plants it feeds on. These usually begin to hatch out during the fall season and a flock of small white grubs get to work devouring the bark around the stem just under the ground. Frequently, in our open climate, they feed all winter long. If the grubs manage to peel the bark completely around the stem, circulation stops to the crown of the bush. As a result, on the first dry, windy day the foliage begins to have a wilted look, much to the distress of the gardener, who wonders why bushes should wilt in soaking wet ground. The bush is doomed; there is no remedy that can save it when damage becomes this extensive. Nor can you find the mischief makers- to punish them, for by this time they have transformed themselves into their chrysalides or have emerged as full-grown beetles and "flown the coop."
The most effective prevention for beetle damage, in my opinion, is to spray all the bushes that they are likely to feed on with arsenate of lead solution, one tablespoon to a quart of water. This poisons the adult so that no eggs will be laid for the next season. Bear in mind, however, that this beetle can fly, and may come into your garden again from elsewhere.
Plants other than rhododendrons may attract it also. It has a predilection for various perennials often grown in gardens, such as the popular garden primulas. In my observation they even seem to prefer Primula roots to rhododendrons for one finds them there much sooner than around the azaleas or rhododendrons. If the primulas are near by, it may be good policy to lure the beetles there, then take up the plants late in November and transplant them to another location. The little maggots of the weevil can easily be found because of their creamy white color. Shaken loose from the primrose roots, they can be destroyed before having done much damage.
Rhododendron growing becomes really difficult in localities where water is hard. Lime is an insidious poison that works slowly on these shrubs. If your soil contains lime, you should plant your rhododendrons in specially prepared beds of lime-free soil. Applications of sulfate of aluminum may help to check alkalinity. It is unwise to give any exact formula, for soils are so varied that only a series of trials will show you what works best. Your ideal proportion might be injurious even in the next door garden, so keep your mind open on the subject of aluminum applications. Also, it has been my observation that overdoses of nitrogenous fertilizers such as cottonseed meal may bring about, a yellowing of the foliage.
Stunting of rhododendron growth may come from lack of drainage or a too-tight soil that remains wet. This condition causes lack of oxygen accessibility to the root system. There are, however, species of rhododendrons able to flourish in heavier soils. The triflorum group with augustinii at its head and the R. heliolepis series seem to thrive in soils too heavy for the average rhododendron, and be fit to grow in full sunshine everywhere. However, for general success, it is wisdom to have porous soils for rhododendrons, and decaying wood seems highly desirable for their root systems.
Rhododendrons, like any other living things, must be given proper care to flourish; but the gardener who cares for his plants will be rewarded with the wealth of beauty that has made this one of our most popular Northwest shrubs.